Titanium dioxide is produced commercially in two crystalline polymorphic forms, namely the rutile form which can be produced by both the chloride and sulfate processes and the anatase form which is produced by the sulfate process. The anatase and rutile forms of TiO.sub.2 differ not only in their crystallographic structures but also in the type and amount of surface and other impurities resulting from their differing methods of production. These differences are such that generally the rutile form produced by the chloride process and the anatase form produced by the sulphate process are processed and refined into finished pigments by significantly different methodologies, with one procedure often not being effective with both forms of TiO.sub.2. This invention is applicable to both the rutile and anatase forms of TiO.sub.2 produced by the sulphate process, but provides especially notable improvements when practiced with anatase TiO.sub.2.
Sulfate anatase TiO.sub.2 is generally produced by digesting a titaniferous ore, such as ilmenite ore, ore concentrates and blends thereof, in concentrated sulfiuic acid to form an iron sulfate-titanium sulfate solution which is subsequently cooled to precipitate out the iron sulfate as copperas. The clarified and concentrated titanium sulfate solution is then heated to hydrolyze and precipitate out hydrous TiO.sub.2. The hydrous titanium dioxide is calcined at an elevated temperature sufficiently high, i.e. at least about 450 to about 1000.degree. C., to partially or completely dehydrate the titanium dioxide to form a water insoluble anatase, or rutile TiO.sub.2 pigment, as the case may be. The crude product resulting from the calcination contains a substantial amount of large particles and grit which must be broken up by either dry or wet grinding.
After dry or wet grinding of TiO.sub.2 pigment to a desired pigmentary particle size, the pigment may be shipped to end users as either a dry powder or an aqueous slurry. The shipment of high solids aqueous slurries has certain advantages to end-users because they do not have to undertake the rather complex and time consuming process of mixing titanium dioxide pigments and water for use in making paper, coatings, paint or other products. However, it also presents disadvantages in that the shipping of large amounts of water over large distances is an inefficient use of resources. Another disadvantage is that the water present in lower solids slurries may create excess dilution in certain applications, such as high solids paper coating, requiring costly and time consuming drying or slowing of paper coating equipment to remove the excess water.
It is known that crude TiO.sub.2 may be compacted and crushed or ground after its production to break-up oversized particles, grit, lumps and aggregates. High solids slurries of anatase TiO.sub.2 containing greater than 73% anatase TiO.sub.2 solids, however, have heretofore not been commercially available.
For instance, U.K. Patent No. 1,204,326 describes subjecting calcined TiO.sub.2 pigment discharge to a pressure rolling step and then a disintegration step which may be a wet milling step. Suspensions of 27 and 35.5% TiO.sub.2 solids and paint containing 37% TiO.sub.2 solids only are described in the examples.
U.S. Pat. No. 5,290,352 discloses the same process as U.K. Patent No. 1,204,326 for preparing inorganic pigment suspensions, including TiO.sub.2 pigment suspensions. Suspensions having solids contents of from 5 to 75% by weight are claimed. However, suspensions of 20 to 25% TiO.sub.2 solids only are described in the examples.
U.S. Pat. No. 5,154,362 discloses a method for crushing brittle material involving first crushing the material in a roller mill to form agglomerates which are then broken up, screened, and passed to a dry or wet mill for further grinding. In U.S. Pat. No. 5,520,340, compacting inorganic pigment on a drum compactor, then de-agglomerating the compacted pigment before it is jet milled is disclosed. In neither patent is the formation of pigment suspensions disclosed.
Resort to more involved processes to achieve high solids TiO.sub.2 slurries have been attempted. U.S. Pat. No. 4,280,849 discloses a process for preparing high solids titanium dioxide slurries in which a first pressure filter cake of a coated TiO.sub.2 is mixed with a dispersing agent, preferably in a sand mill, then dewatered under super atmospheric conditions to produce a second filter cake that is reslurried to form a high solids slurry of 60 to 75% coated titanium dioxide. Similarly, U.S. Pat. No. 4,978,396 discloses a method said to be useful in obtaining 70 to 80% solids slurries of rutile or anatase titanium dioxide in which an initial low solids slurry of wet milled and hydroclassified pigment is dewatered under super atmospheric conditions to produce a wet filter cake that is reslurried to form the high solids slurry. The lone example of this patent obtained a solids content of 71%.
U.S. Pat. No. 4,288,254 discloses a process for preparing a high solids slurry of rutile TiO.sub.2 by passing a slurry of wet milled or, dry micronized, dispersant-coated TiO.sub.2 through a high sheer mill to de-agglomerate the TiO.sub.2. The sole example shows the preparation of a slurry having at most 64.6% solids.